1. Field of the Invention
The present invention relates to an antenna feed arrangement for correcting for astigmatism caused by the main reflector of an antenna.
2. Description of the Prior Art
When a feed is displaced from the focus in a reflector antenna, the loss in efficiency is due primarily from astigmatism which is found to be a different amount at each location of the feed on the focal surface. Antenna systems have been previously devised to correct for certain aberrations including astigmatism.
U.S. Pat. No. 4,145,695 issued to M. J. Gans on Mar. 20, 1979 relates to launcher reflectors which are used with reflector antenna systems to compensate for the aberration of astigmatism which was found to be introduced in the signals being radiated and/or received at the off-axis positions. A major portion of such phase error is corrected by using, with each off-axis feedhorn, an astigmatic launcher reflector having a curvature and orientation of its two orthogonal principal planes of curvature which are chosen in accordance with specific relationships, the launcher reflector being fed by a symmetrical feedhorn.
U.S. Pat. No. 4,339,757 issued to T-S. Chu on July 13, 1982 and U.S. Pat. No. 4,343,004 issued to E. A. Ohm on Aug. 3, 1982 each relate to an antenna arrangement capable of correcting for astigmatism over a broadband range. Each of the patented arrangements comprise a main focusing reflector arrangment, a feed arrangement and an astigmatic correction means disposed between the feed arrangement and the main focusing antenna arrangement. The astigmatic correction means comprises a first and a second doubly curved subreflector disposed in a predetermined manner in the antenna arrangement and which are curved in orthogonal planes to permit the launching and reception of an astigmatic beam of constant size and shape over a broadband range.
The prior art arrangements, however, only compensate for astigmatism for a predetermined off-axis position on the focal surface. Therefore the problem remaining is to provide astigmatic correction feed arrangements which can be easily reoriented on the focal surface of an antenna arrangement and can be adjusted to cancel astigmatism at the new location.